Alkali-free ceramic materials and method of making same



Patented Jan. 26, 1937 UNITED STTES PATT OFFEQE ALKALI-FREE CERAMIC MATERIALS AND METHOD OF MAKING SAME Albra H. Fessler, Flint, Mich" assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application December 14, 1935,

Serial No. 54,428

8 Claims. (Cl. 23-142) This is a continuation in part of my prior apin the electric furnace and have continued treatplication Serial No. 729,345, filed June 6, 1934. ment until the analysis indicates complete ab- This invention has to do with a method of prosence of boric acid and reduction of alkalies to ducing ceramic material, especially non-plastics, a trace.

of very low alkali content as well as with the In the case of zircon and titanium oxide Ihave 5 resulting products. employed calcination with the same results in In certain commercial uses of ceramic matereduction of alkali. rialsit is very desirable that the alkali content In the case of magnesium oxide I have embe aslow as possible. Thus in the manufacture ployed both calcination and fusion with satis- 10 of high grade spark plug porcelains the presence factory results. 10

of alkalies is objectionable because they have The utility of the method in the treatment of the effect of reducing the hot dielectric strength various non-plastics including silicon carbide, of the insulator. In the case of ceramic supports corundum, etc., is indicated.

and spacers for the emitting and heating ele- An especially important application of my ments of vacuum tubes, the presence of alkalies method is in the manufacture of alumina. By 15 is also seriously objectionable as they tend to my method alumina may be cheaply produced volatilize, raising the gas pressure in the tube, having an alkali content lower than the variety producing undesirable space charge effects and now designated as C. P. shortening the life of the electrodes through posi- A method heretofore commonly employed in tive ion bombardment. the manufacture of alumina of low alkali content 20 By the process herein disclosed it is possible is as follows: Bauxite, (A12O32H2O) is heated to produce at low cost ceramic materials of exwith sodium carbonate (NazCOs), producing sotremely low alkali content. dium aluminate NaAlOz or NazO, A1203. Water In general the process consists in adding boric is passed through the resulting mass, resulting in oxide (B203) to the raw material and heat treatthe production of a solution of sodium alumi- 5 ing it, as by fusing or calcining, for a sufiicient nate, and leaving a residue of finely divided oxide length of time to drive off both the boric acid and of iron. From the solution aluminum hydroxide the major portion of the alkaline impurities. Al(OH)3 or A1203, 31-120 is precipitated by the Analyses of the resulting product indicate entire action of carbon dioxide at 50-60":

elimination of the boric oxide and the reduction 30 of alkalies to a very small amount, at times a 2NaA1O2+CO2+3H2O Na2CO3+2Al(OH)3' mere trace. The precipitate is filter pressed and washed with I have successfully applied the process to a water to remove most of the sodium carbonate. number of non-plastics including alumina, mul- Thereafter in order to further reduce the alkali 35 lite, magnesia, zircon and titanium oxide, all of content the filter press cake is washed four or 5 these materials having possible use in the manufive times with acid. The resulting purified facture of spark plug insulators. In each case aluminum hydroxide is then calcined at around I have preferably first determined the amount 1,000 C., which drives off most of the water, of alkali present in the material and have then leaving aluminum oxide. Such specially treated added an amount of boric acid suflicient to rematerial regularly contains as much as .3% al- 40 move it. Thus in cases where the predominant kali content, compared with .6% in the case of alkali is soda, boric acid has been added in the standard alumina, and .13% in the case of C. P. molecular ratio of 413203 to lNazO, or 280 units alumina. by weight of boric acid to 62 units by weight of In carrying out the present invention the above NazO. described process is followed down to the point of 45 The HOH-PIBJStiC. mixed With boric OXide, 11Sllwashing the filter press cake with acid. This ally added as boric acid, is then heat treated step is eliminated. Instead, the aluminum hyeither by fusion in the electric furnace or by caldroxide is dry mixed with boric oxide (H3303) cinatiOfl a OW temperatures, 011 the Order just before it is introduced into the rotary kiln of 1400 C. Where the material is fused it is for calcination. The amount of boric oxide re- 50 not necessa y to thoroughly mix the raw matequired is determined by the alkali content. Thus rials but in the case of calcination the powdered for every 62 units by weight of NazO in the ingredients should be thoroughly mixed to promaterial treated 280 units by weight of B203 mote the action. should be used. This is in the molecular ratio In the case of mullite I have employed fusion of lNazO to 4B2O3. While the principal effect of 55 the boric oxide is to remove soda, it is believed that it also eiiects a reduction of the alkalies in general.

While the usual calcination temperature is about 1,000 0., as previously stated, and at this temperature some of the soda is volatilized, for eifective removal of the soda, calcination temperatures in the neighborhood of 1300 C. are required. However, the higher temperature alone without the addition of boric acid will not accomplish the removal of the soda although higher calcination temperature does affect the total acid soluble, but this in turn does not affect the total alkali content. In other words, the higher calcination temperature does change the lowtemperature form of alumina, which is known as gamma, to the high temperature form of alpha, but this change does not promote the loss of soda during calcination.

Alumina purified by boric oxide treatment is found to contain a total alkali content as low as 08% and at times as low as .05%. The purified product costs no more, if as much, to produce as the acid washed alumina with its 30% alkali content, for the repeated washing with acid is eliminated and is replaced with the simple dry mixing of aluminum hydrate with boric oxide combined with calcination at slightly higher temperatures. By the new process the possibility of contamination of the alumina by the action of acid on the washing equipment .is, of course, eliminated. A sample of the new, purified alumina, analyzed in our laboratories showed the total alkali content to be .08%, acid-soluble alkali 114% and the total acid soluble content 26%.

It is to be expected that many other uses will be found for the purified products.

I claim:

1. The method of producing ceramic materials substantially free of alkali which consists in adding boric acid to ceramic material containing alkaline impurities and heat treating the material to an extent to volatilize the boric acid and with it the major portion of the alkali.

2. The method of producing ceramic materials substantially free of alkali which consists in determining the amount of alkali present in the material, adding thereto boric acid in the molecular ratio required to completely reduce the alkali, and heat treating the resulting mixture until the boric acid is volatilized together with the major portion of the alkali.

3. The method of making alumina of low alkali content which consists in mixing aluminum hydroxide containing alkaline impurities with boric acid and calcining the mixture at temperatures below the temperature of fusion and con tinuing such calcination until the alkaline impurities are volatilized along with the boric acid.

4. The method of making alumina of low alkali content which consists in mixing aluminum hydroxide with boric acid in substantially the proportion of at least four molecules of boric purities are volatilized along with the boric acid.

6. The method of making alumina of low alkali content which consists in heating bauxite with sodium carbonate, passing water through the resulting mass, subjecting the consequent solution to the action of carbon dioxide, mixing the resultant precipitate with boric acid and calcining the mixture to volatilize the alkali content.

7. The method of making alumina of low alkali content which consists in heating bauxite with sodium carbonate producing sodium aluminate,

passing water through the resulting mass pro 

